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Think of the Chaiten Volcano which erupted recently in Chile with that huge plume of gas and rock and put it underwater.
Analysis of some of the shards showed them to be bits of glass known as limu o Pele, or "Pele's seaweed". These are formed when lava is stretched thin around expanding gas bubbles during an explosion. Scientists collected rock and sediment samples and high definition videos showed shards and bits of basalt over the seafloor with some deposits on top of new lavas an indication the debris had fallen rather than been moved as part of a lava flow.
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For the climate change deniers, this volcanic activity has nothing to do with the Polar ice melting faster than usual. The stratified waters of the Arctic Ocean can absorb and disperse any heat and gas from an eruption on the seafloor. Cold dense water in the depths are trapped near the bottom with little mixing with warmer surface layers. Almost no heat is transmitted to the underside of the Polar ice from approximately 3,ooo to 4,000 metres below.